A fundamental device used in optical communication systems is the optical coupler, a device described, for example, in the U.S. Patent of Dl Domenico, et al. 4,165,496, a patent assigned to a subsidlary of the present assignee. The coupler basically comprises two end-to-end optical fibers having abutting ends that are cut at an angle and coated with a partially reflecting coating such as a multilayer dielectric or a metallic film. The angled interface between the two optical fibers then constitutes a partially reflecting surface by which a portion of the light propagating along the two fibers can be removed. The removed portion can be used for a number of purposes, such as controlling the original light source, controlling a repeater, etc. The interface can also be used to introduce light wave information onto one of the fibers.
It is apparent that the two abutted optical fibers must be in precise alignment for the coupler to work properly. Such alignment is normally assured by the use of a silicon substrate having a V groove in one surface for supporting the two abutting fiber ends. Often, a light wave system requires a plurality of couplers for a plurality of parallel fibers, which can be implemented through the use of a plurality of side-by-side V grooves in the silicon substrate. The plurality of optical fibers are then located side-by-side in the V grooves, and separate photodetectors are used to detect the partially-reflected output from each coupler.
Silicon is a good material for making the small grooves needed for fiber support and alignment because known principles of photolithographic masking and etching can be used to define the grooves. Because of the crystalline structure of silicon, one can employ anisotropic etching to form the V-shaped troughs with great precision; this is important because each fiber has a diameter of only about 125 microns.
One problem with using a silicon substrate for this purpose is that silicon has a different index of refraction from that of glass and a different coefficient of thermal expansion. The difference of thermal expansion of the glass optical fibers and the silicon substrate can cause small but harmful misalignments and can restrict the selection of adhesives. The difference of index of refraction between the silicon and the glass can cause unwanted reflections.